Kathleen Sebelius, Secretary of Health and Human Services, testifies to the Senate Finance Committee in Washington March 16, 2011.

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U.S. Veterans Affairs Secretary, retired general Eric Shinseki makes remarks during a visit to the Coatesville VA Medical center in Coatesville, Pennsylvania, April 12, 2012.

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WASHINGTON (Reuters) - Labor Secretary Hilda Solis, the first Latina to head a major federal agency, announced plans to resign on Wednesday, just as President Barack Obama faces questions about a lack of women in his second-term leadership team.

Solis said in a statement she had submitted her resignation to Obama - not an unexpected development given that many Cabinet members grow fatigued by the long days in office and are eager to look at opportunities in the private sector after four years.

She grappled with trying to bolster the U.S. workforce at a time of fiscal crisis and recovery from recession. "There is still much to do, but we are well on the road to recovery, and middle-class Americans know the president is on their side," she said.

Obama, in a statement, called her a "tireless champion for working families."

So far, Obama has picked white males for the three biggest Cabinet positions - Senator John Kerry to replace Hillary Clinton as secretary of state and former Senator Chuck Hagel as defense secretary. He was due to announce Jack Lew as treasury secretary on Thursday, a source said.

Susan Rice, the U.S. ambassador to the United Nations and a close confidante of Obama, withdrew her name last month from consideration for the secretary of state post in the face of what promised to be a contentious Senate confirmation battle.

Rice had drawn heavy fire from Republicans for remarks she made in the aftermath of a September 11 attack on the U.S. mission in Benghazi, Libya, in which four Americans were killed, including Ambassador Christopher Stevens.

The appointments of men only are leading to some questions as to whether Obama is losing diversity from his senior leadership team.

White House spokesman Jay Carney urged critics to render a judgment after Obama has filled out his team.

"Women are well represented in the president's senior staff here," he said, noting that two of the president's deputy chiefs of staff are women, as is the White House counsel.

At the same time as the Solis announcement, the White House said that Attorney General Eric Holder would be among first-term Cabinet members who will stay as Obama begins his second term. Holder, the first black U.S. attorney general, has been a lightning rod for criticism from conservatives.

Republicans tried to oust him after a botched department operation called "Fast and Furious" that targeted gun trafficking along the U.S.-Mexico border.

Two others Cabinet members plan to remain as well: Health and Human Services Secretary Kathleen Sebelius and Veteran Affairs Secretary Eric Shinseki, a White House official said. There appeared to be no plans for Homeland Security Secretary Janet Napolitano to leave.